COVID-19 vaccine registration is open to everybody aged 44 from today.
Those aged 40-43 will then be able to register over subsequent days - 43-year-olds tomorrow (Thursday June 3rd), 42-year-olds on Friday etc...
It's expected vaccination appointments for the age group will then be scheduled from the middle of the month.
Registration can be done via the HSE's online portal or by phone, with everybody over 44 still able to register if they haven't already.
Gerald Barry, Professor of Virology at UCD, said the further extension of the vaccine programme is good news.
He said: "The vaccination programme, in general, has been really good, I think. Targeting the age cohorts seems to have been the right approach in terms of speed.
"There have been some people who've slipped through the gaps, and hopefully there's a concerted effort to pick those people up.
"In general, though, it has been really high uptake - people have been rolling their sleeves up and getting vaccinated."
It was confirmed earlier this week that more than half of Irish adults have now received at least one dose of a COVID-19 vaccine.
The HSE's Dr Colm Henry has suggested the vaccine rollout in Europe has been 'the envy of Europe' due to high levels of uptake.
Officials are continuing to experience some issues around vaccine supply, with a 'significant under-delivery' of the one-dose Janssen jab expected this month.